Stone
Cantilevered Stairs

Russell
Taylor

The Exhibition Staircase at Somerset House (Sir William Chambers, 1775-1795). An 18th century guidebook noted that 'the stair, though winding, is easy and covenient. It has the merit of being contained in a very small space, without prejudice either to its commodiousness or magnificence'.

Stone
cantilevered staircases are not uncommon. Everyone has seen one,
usually an elegant, flowing sweep of stairs in an 18th century
building, and yet they are still intriguing. Why, you might ask,
are there so few 19th century examples in England? And how could
a brittle piece of stone project 1,200 or 1,500mm out from a wall
without snapping in two? The answer to the latter is that the
stone treads are not cantilevers. The term is a misnomer although
it remains in common usage.

HISTORY

The first
stone cantilevered staircase constructed in England is believed
to be Inigo Jones' Tulip Staircase in the Queen's House at Greenwich
1629-35 (below right). It is based on the work of Andrea
Palladio who, 60 years earlier, had illustrated various forms
of spiral staircases in the first book of his Quattro Libri, which
explored the virtues of ancient Roman architecture and inspired
generations. In particular he mentioned a classical example which
is 'void in the middle' at 'Santa Maria Rotunda' (the Pantheon),
Rome. 'Winding staircases', he wrote, 'succeed very well that
are void in the middle, because they can have the light from above,
and those that are at the top of the stairs see all those that
come up or begin to ascend, and are likewise seen by them'. He
continued
reassuringly; 'I have made a staircase void in the middle, in
the monastery de la Carita in Venice, which succeeds admirably'.
He thought that stone cantilevered stairs were elegant examples
of Roman work, and used them in his designs in continuation of
the classical tradition.

Inigo Jones was not generally well served
by his craftsmen in the Queen's House, but the craftsmanship of
the Tulip Staircase is exceptional. His mason, Nicholas Stone,
a craftsmen of great ability and a fine architect is his own right,
understood perfectly how to make a cantilevered staircase: not
only did he have the visual knowledge that Jones had gained through
a study of the works of Palladio, but he also realised how to
make it work structurally. Whereas Palladio's cantilevered stairs
have treads that merely overlap, in the Tulip Staircase Nicholas
Stone formed a 50x38mm rebate along the bottom of the riser -
not an obvious or an easy thing to do because the
coursing of the stonework is interrupted. This rebate is an important
feature of the structural design of a stone cantilevered staircase
(see Figure 1). How did Nicholas Stone know of its importance?
As far as we know, Stone's use of the rebate was the first in
the world, a development crucial for more daring stairs.

Above: the Navy Office Stair at Somerset House (Sir William Chambers, 1775-1795). The central flight is supported by an arch beneath. Below right: the Tulip Staircase at the Queen's House in Greenwich (designed by Inigo Jones and executed by Nicholas Stone in the 1630s)

Ultimately,
the riser rebate (which is also known as an 'interlock' or, in
Scotland, a 'pencheck')
allowed the construction of such great staircases as that of Chatsworth
House (1688-91) which projects a stupendous 2.15m out of the wall,
compared to the comparatively modest 1.25m of the Tulip Staircase.
At Chatsworth the staircase is not a spiral but a series of straight
flights connected by quarter
landings. With spiral stairs one can, perhaps, imagine all the
loads spiralling down the free ends to
the ground, but
it takes even greater imagination to see how this would work with
straight flights and quarter landings.

The subsequent history
is easily told. During
the 18th century,
staircase design became increasingly lighter and more elegant.
It also became more daring. It was as if the designers' goal was
to have no visible means of support. Until it was arrested by
the Victorian Gothic revival, this was the inexorable direction
of staircase
design. The
stairs
became a way of implying movement between the static levels of
each
storey
in a classically designed building. The
gravity-defying display
of William Chambers' Navy Staircase in Somerset
House is an
inspirational
example (below).

Generally the individual stone
treads were rectangular in section in the 17th and early 18th
century, then modillion
shaped
in the mid and late 18th (giving a rippling soffit) and finally
triangular in section in the late 18th century and Regency. Balustrades
followed
a similar route ending with simple wrought iron verticals, as
favoured by Sir John Soane, for example.

STRUCTURAL
DESIGN

An inspection
of cantilevered stairs should convince anyone that they are not
in fact cantilevering. If they were, the treads would be tailed-in
quite a long way, but they are not. The Geometric Staircase in
the west front of St Paul's Cathedral is tailed-in 150mm, the
staircases at Hampton Court Palace, 225mm, and most early 19th
century examples are tailed-in half a brick or 115mm.

This principle
is further demonstrated in many London town houses. Often there
is a stone cantilevered staircase connecting the main floors and
a secondary timber staircase serving the upper floors. Where the
stone staircase finishes (usually at second-floor level) the stairwell
wall is continued above the level of the top tread as a timber
stud wall. Therefore the tailing-in is not used to counteract
a cantilever force.

So how does it work? First of all consider
plain unrebated treads. At a typical tread in a flight the back
edge carries the weight of those above and its front edge is supported
by those below. The wall carries
the torsion resulting from these two opposing forces (see Figure
1). Clearly the weight increases down the flight. The first (bottom)
tread is supported by the floor, so it is the second tread which
carries the greatest torsion.

Unrebated treads are usually only
found in basement stairs. And it is not difficult to see why:
they are often not very wide, they are almost always rectangular
in section (and therefore fairly strong in torsion) and sometimes
supported by a wall under the free end.

The majority of stairs
have riser rebates. Consider a typical tread in Figure 1: any
clockwise movement will be resisted by the tread of the lower
step and by the soffit of the upper step; any counter-clockwise
movement will be resisted by the vertical face of the rebate in
the step above and by the back of the step below where it interlocks
with the rebate. There are then horizontal forces acting in opposite
directions at the top and bottom of the tread. This reduces the
torsion in the stone considerably. The stresses are also reduced
because the rebate allows the torsion to be carried across the
full length of the tread, rather than solely by the restraint
from the supporting wall.

Figure 1: Forces in steps without a rebate (left) and with one (right)

Figure 2: Modillion-shaped treads were fashionable in the mid to late 18th century (left). Simple triangular sections (right) became more common in Regency staircases.

There are other important advantages
of the rebated detail. It allows the treads to act together so
that a weakness, or even failure, in one, does not bring about
the failure of the whole flight. It also allows the staircase
to cope better with any movement that might occur in the supporting
wall for some quite independent reason, such as subsidence.

The
actual dimensions of the rebate, both horizontally and vertically,
do not appear critical, although the smaller they are, the more
important the workmanship. Historically, they have been as small
as 12mm.

The riser rebate is the key to the design of the most
elegant stone cantilevered staircases. Nicholas Stone understood
its importance for the Tulip Staircase, and clearly this technique
was generally understood and widely used in the period that followed.
The exact source of the spark of inspiration or enlightenment
which resulted in the riser rebate remains a mystery.

The structure
of landings is a little more complicated. As well as being connected
to the stair, a landing may span from one side of the stairwell
to the other, and in this case its stone slab is clearly not cantilevered.
In other cases it may be partially cantilevered where supported
across less than half of its circumference or across a corner.
But often the slab will be thinner than the treads and therefore
inherently less strong. In large and long landings
some or all slabs may be potential cantilevers, so they are usually
supported on projecting console brackets.

Above left: Lower flights of the Stamp Office staircase at Somerset House (Sir William Chambers, 1775-1795). The shortness of the flights, the large area of the rectangular treads in section and the comparatively slim width of the stairs meant that it was not necessary to rebate the profile. Torsion is still resisted by the wall. Further up, as the staircase becomes more elaborate, a rebate for the treads becomes necessary to make up for the diminished area of the section. Above right: the Stamp Office Staircase, upper flights. Note that the landing (top left of picture) is slimmer than the overall depth of the tread.

CONSERVATION
AND REPAIR

Once a stone
cantilevered staircase has been properly constructed, there are
generally only three possible causes of failure:

accidental
damage to an individual stone tread or group of treads

a loss
of rigidity either in the wall or between the treads

weakening
by the cutting-in of new nosings.

Accidental damage to an individual
stone tread (or a group of treads) may lead to cracking or the
loss of a piece of stone beyond the crack. The cause is usually
a sudden impact at a concentrated point, such as dropping a piece
of heavy furniture onto the stairs. Stone stairs are particularly
vulnerable during building work, and a point load from scaffolding
should never be applied, even if also supported from underneath.
Damaged stairs will not be significantly weakened provided more
than a quarter of the tread remains intact at the wall end. Cracks
can be stitched together and lost pieces fixed back in place by
the insertion of stainless steel dowels and the injection of resin
around the dowels and in the cracks.

Cantilevered staircase under construction over temporary blockwork supporting walls. The treads slot into the brick's width cavity in the side wall. (Photo: CWO Ltd)

A loss of rigidity, either
in the wall or in treads may lead to excessive movement between
the treads. Stone cantilevered stairs are the exception to the
traditional construction rule that everything must be flexible.
Here everything must be rigid and the treads must be tightly embedded
since if there is excessive movement, particularly where the steps
are built into the wall, the torsional forces will not be restrained.
In an existing staircase it is probably easiest to ensure that
this is achieved by resin injection. Open and cracked joints between
treads should be raked out, cleaned and repointed to ensure that
every tread is properly interlocking with those above and below.

New nosings are often required to counteract excessive wear in
stone treads. One common solution is to cut back the steps to
a new line, but in a cantilevered stair this is dangerous and
should never be done. This is because the cross-sectional area
of each wedge-shaped tread is of structural significance, and
cannot be reduced without impairing its performance. Where treads
have been cut back in the past, look for cracks in the treads
which coincide with the end of a cut-in nosing. These are a sign
that the cross sectional area is insufficient to resist the torsion
in normal use. If just a few treads are cracked like this, the
stairs can be repaired by stainless steel stitching and resin
injection. However, if many are cracked the stairs may need to
be supported by a raking stringer (in effect an inclined beam
following the profile of the staircase from top to bottom), but
this should only be considered as the very last resort.

If worn
or chipped nosings really do need attention they can be repaired
and covered with a carpet or built-up back to the original profile
in a resin-based mortar. A rather better solution is to keep the
worn nosing and improve safety by applying adhesive non-slip strips.
These have the benefit
of being non-invasive and completely reversible.

If none of these
three structural problems is evident (that is to say, that there
are no cracks, no movement and no cut-in nosings) then the staircase
is sound. The aim should then be to keep it that way and not allow
disfiguring props and stringers to be introduced unnecessarily,
or simply because the structural principles are not understood.

This
article is reproduced fromThe
Building Conservation Directory, 2006

Author

RUSSELL TAYLOR is the
principal of Russell Taylor Architects, a practice specialising in classical
design. In addition to the conservation of stone cantilevered
stairs, he advocates their use in new buildings as a practical
and elegant alternative to concrete.